1. Field of the Invention
The disclosed and claimed invention generally relates to local area networks, and is particularly directed to a transceiver which includes circuitry for detecting the presence of two or more data signals on the communications channel of a local area network.
2. Background Art
The ever increasing use of electronic information processing devices and their high speed processing capabilities has created a need for high speed data communications within a local area of limited geographic scope, such as a building or a small campus. Efforts to meet the need have produced a variety of networking schemes referred to as local area networks or LANs. The known LANs differ in their characteristics, including transport technique, transmission medium, topology, and access protocol.
The disclosed invention is for use with a local area network which utilizes baseband techniques for data communication over a common data bus such as coaxial cable. In such a bus configuration, the coaxial cable is of limited length (e.g., 600 meters without a repeater) and is appropriately terminated at both ends. Individual station equipment such as computers, video display terminals and storage device access the cable via a Bus Interface Unit (BIU) and a Bus Access Module (BAM). The station equipment is connected to the BIU which implements the particular protocol of the local area network. The BIU may be embedded within the station equipment and is coupled to the cable through a BAM which includes among other elements a transceiver. The BAM is typically coupled to the cable with a high impedance transmission tap. A station and its associated BIU and BAM collectively are sometimes referred to as a terminal.
In order to insure proper receipt of messages, only one message can be on a local area network bus at any given time. Therefore, protocols have been developed to insure that a given message can be transmitted and received.
One widely used protocol is referred to as Carrier Sense Multiple Access/Collision Detect (CSMA/CD) wherein a terminal wanting access to the data bus first determines whether the data bus is being used. If the data bus is being used, the terminal waits until the transmission is completed. Otherwise, the terminal begins to transmit. With the CSMA/CD protocol, the transmitting terminal must detect collisions caused by at least two terminals transmitting simultaneously. Generally, a transmitting terminal detects collisions by monitoring the data bus to determine whether the message on the bus agrees with the message it transmitted. If it does not agree, then some form of collision recovery procedure is implemented.
Another protocol technique is known as token passing wherein the token, essentially a codeword, is sent sequentially from one terminal to another. Only the terminal possessing the token is permitted to transmit. After the token possessing terminal finishes transmitting or if it has nothing to transmit, it transmits the token for reception by the next terminal in a predetermined sequence. To the extent that the next terminal does not accept the token within a predetermined time interval, such terminal is assumed to be inactive or non-existent. The appropriate protocol procedure is then implemented for transmitting the token to next terminal after the non-responsive terminal.
With either type of protocol, each terminal must detect (i) the occurrence of collision, and (ii) activity on the data bus. While token passing is designed to avoid collision, collision detection must still be utilized with a token passing protocol since collision may still result from the addition of new teminals, malfunctions, and/or transmission errors.
With networks utilizing long cables and/or wideband data signals, detection of collision by a transmitting terminal is difficult where the contending transmission is a signal from a remote distant terminal, since the contending signal from the remote distant terminal is considerably attenuated while the data signal of the local transmitting terminal has a high level. Such attenuation of the data signal from a remote distant terminal results from the attenuation properties of the data bus.
Known techniques for collision detection include detecting the presence or absence of the ac components of two or more simultaneous baseband data signals. However, detection of a very low level contending signal from a distant remote terminal is unreliable.
Another known technique for collision detection involves detection of the dc component of two or more simultaneous baseband data signals. This technique requires dc coupling between each terminal and the data bus if message length is not limited. However, dc coupling can result in circumstances where the failure of one coupler may affect the couplers of other terminals.
A known technique for activity detection involves detection of the presence or absence of baseband data signals. However, with such technique reflections may be erroneously detected as busy activity. Erroneous detection of bus activity would prevent utilization of the cable and perhaps unnecessary implementation of collision recovery procedures since bus protocols typically depend upon accurate activity detection for proper operation.
Thus, in general, present data bus type local area networks required complex circuitry to detect collisions and activity, and such circuitry becomes unreliable under known circumstances.
Without the use of repeaters, the complexity of collision/activity detection presently limits the number of terminals which a local area network can accommodate and the length of cable for such a network. While the use of repeaters allows a local area network to accommodate more terminals and a larger data bus, the use of repeaters significantly reduces the reliability of the data bus.